I note the kind contribution of Richard Chonak who translated the Latin original of the second edition (1988). The deletion of some observances (like the Pentecost octave) meant some chants were unassigned in the new calendar. However, the expansion of two cycles of readings meant a new opportunity to assign these chants plus some others to the Church’s liturgy.
Above all, care was taken to preserve the authentic Gregorian treasury in its integrity. Accordingly, chants pertaining to Masses which no longer had a place in the liturgical year were assigned to form other Masses (for example, for the weekdays of Advent, for weekdays between Ascension and Pentecost), or were substituted for others that occurred more than once in the year. (for example, in Lent or on Sundays of Ordinary Time), or if appropriate to their character, were assigned to the celebrations of Saints.
As well, about twenty authentic Gregorian texts, which had been taken out of the repertory, through various changes made in the course of time, were restored to it. Provision was made that none of the authentic chants were disfigured or mutilated, excepting certain elements that perchance were inappropriate to the liturgical season, such as Alleluia, which occurred in the text of antiphons with notes that constituted an integral part of the melody.
There was clearly a lot of care on two fronts: ensuring that the fullest repertoire was available, and that the music would be put at the service of the Church’s liturgy, not vice versa.